The present invention relates to a novel Ni-base superalloy to be used as a material for members of apparatus operating at a high temperature, such as a bucket and/or a stationary vane of gas turbine, especially, to a superalloy preferable as a material for members to be used at a high temperature, composed of a single crystal alloy having a superior strength at a high temperature, and also having a large scale complex shape which is difficult to manufacture with a high productionyield for conventional single crystal alloy.
A combustion temperature of gas in gas turbines have tended to increase every year with the aim of improving thermal efficiency, and accordingly, a material having a strength at a high temperature superior to conventional material is required as the material for respective members of the gas turbine operating at a high temperature. For instance, the material for the bucket and/or the stationary vane which is exposed to the severest environment among the members of gas turbine operating at a high temperature, has been shifted from conventional castings of Ni-base superalloy to columnar grained castings. Further, a single crystal material having a high temperature strength is practically used in a gas turbine for engines of aircraft. The columnar grained material and the single crystal material are kinds of so-called directionally solidified material, and both of the material are cast by a method known as a directionally solidification method. The high temperature strength of the columnar grained castings can be improved by growing crystal grains slenderly in one direction by the method disclosed in U.S. Pat. No. 3,260,505, and others, in order to decrease the number of grain boundaries perpendicular to the direction of an applied main stress to as few as possible. The high temperature strength of the single crystal castings can be improved by making the whole cast body a single crystal by the method disclosed in U.S. Pat. No. 3,494,709, and others.
In order to improve further the high temperature strength of the Ni base superalloy, a solution heat treatment for precipitating .gamma.' phase, i.e. a precipitate strengthening phase, finely and uniformly in the superalloy is effective. That means, the Ni base superalloys are strengthened by precipitation of the .gamma.' phase composed of mainly Ni.sub.3 (Al, Ti, Nb, Ta), and the .gamma.' phase is desirably precipitated finely and uniformly. However, when the superalloy is in a solidified condition without any treatment, coarse .gamma.' phases (a .gamma.' phase which was precipitated and grown during a cooling period after the solidification and eutectic .gamma.' phases which were formed coarsely at a final solidified portion) exist. Therefore, the high temperature strength of the superalloy can be improved by the steps of heating the superalloy to dissolve the .gamma.' phase into the base .gamma. phase, then cooling rapidly (a solution heat treatment), and precipitating fine and uniform .gamma.' phase during subsequent aging heat treatment. The solution heat treatment is desirably performed at a temperature exceeding the solves temperature of the .gamma.' phase, and at as high a temperature as possible below the incipient melting temperature of the alloy; because the higher the heat treatment temperature is, the wider the region of fine and uniform .gamma.' phase becomes.
Further, the wider the region of fine and uniform .gamma.' phase is, the more the high temperature strength of the superalloy is improved. Another reason of the superior high temperature strength of the single crystal castings is that the temperature for the solution heat treatment can be increased by using an alloy exclusively for forming a single crystal, containing chemical elements for grain boundary strength which lower significantly the incipient melting temperature of the alloy by a very small amount such as an impurity level, and consequently, almost all the .gamma.' phase precipitated coarsely after the solidification can be made fine and uniform.
As explained above, the single crystal castings of the Ni base superalloy is the most superior material for the material of bucket and/or stationary vane of gas turbines in conventional technology. Therefore, single crystal alloys such as CMSX-4 (U.S. Pat. No. 4,643,782), PWA1482 (U.S. Pat. No. 4,719,080), Rene' N5 (JP-A-5-59474 (1993)), and others have been developed, and used practically as the material for a bucket and/or a stationary vane of gas turbines of aircraft engines. However, as explained above, these single crystal alloys contains chemical elements such as C, B, Hf, and the like for grain boundary strength by only an impurity level. Accordingly, if any grain boundary exists in the bucket and/or the stationary vane cast from the single crystal alloy, the strength of the bucket and/or the stationary vane decreases extremely, and in some cases, a vertical crack is generated in the bucket and/or the stationary vane along the grain boundary during the solidification step. Therefore, when the bucket and/or the stationary vane cast from the single crystal alloy is used for the gas turbine, the whole bucket and/or the stationary vane should be a complete single crystal. Because the bucket and/or the stationary vane of the gas turbine for aircraft is approximately 100 mm long at the maximum, the probability to generate a grain boundary during the casting is small, and the bucket and/or the stationary vane of single crystal alloy can be produced with a reasonable production yield. However, as the bucket and/or the stationary vane of the gas turbine for power generation is approximately 150.about.450 mm long, it is very difficult to produce the whole bucket and/or the stationary vane with a complete single crystal. Accordingly, with the conventional technology, it is difficult to produce the bucket and/or the stationary vane of the gas turbine for power generation using the conventional single crystal alloy with a reasonable production yield.
In order to improve the strength at a high temperature of large size bucket and/or stationary vane, for which the single crystal alloy can not be applied in view of a low production yield at the casting process, development of alloys for columnar grained castings having a preferable strength at a high temperature was performed, and as the result, the Ni base superalloys for columnar grained castings such as CM186LC (U.S. Pat. No. 5,069,873), Rene' 142 (U.S. Pat. No. 5,173,255) were developed. These alloys have a sufficient amount of chemical elements for preventing generation of solidification cracks, and ensuring a sufficient reliability during operating time, and concurrently, have a high temperature strength comparable to the single crystal alloys of the first generation such as PWA1480 (U.S. Pat. No. 4,209,348), CMSX-2 (U.S. Pat. No. 4,582,548), Rene' N4 (U.S. Pat. No. 5,399,313), and the like. Therefore, it became possible to produce the bucket and/or the stationary vane having approximately the same strength as the bucket and/or the stationary vane made of the first generation single crystal alloy at a high temperature with a reasonable production yield by using these alloys for columnar grained castings. However, currently, the strength at a high temperature of these conventional alloys for columnar grained castings has become insufficient for satisfying a requirement to improve further a thermal efficiency of gas turbines, because a combustion temperature of gas turbines has been in a tendency to increase further.
The single crystal alloys having columnar grains containing C, B, Zr, and Hf are disclosed in JP-A-7-145,703 (1995) and JP-A-5-59,473 (1993).
In view of the above described aspect of the prior art, development of an alloy, wherein a high production yield and a high strength at a high temperature, which are conventionally deemed as contradictive, are compatible with each other is regarded as indispensable for improving the efficiency of the gas turbines for power generation.
As previously described, a method to make the heating temperature in the solution heat treatment as high as possible is effective for improving the high temperature strength of the Ni base superalloy, and the additive amount of the chemical elements for grain boundary strength is preferably as small as an impurity level therefor. On the other hand, in order to ensure a high production yield and a high reliability during operating time, the chemical elements for grain boundary strength to give an appropriate strength to the grain boundary should be contained in the superalloy. Therefore, conventionally, the strength at the grain boundary had to be sacrificed in order to improve the high temperature strength, and on the contrary, the high temperature strength had to be sacrificed in order to improve the strength at the grain boundary.
In accordance with the study performed by the present inventors on the conventional alloys for columnar grained casting, i.e. CM186LC (Material and Process Vol. 7 (1994), p1797, and ibid Vol. 8 (1995), p1458), it has been revealed that B, one of the chemical elements for the grain boundary strength, diffuses from the grain boundary into inside grain during the solution heat treatment. Accordingly, although the alloy contains the chemical elements for grain boundary strength, the strength at the grain boundary of the alloy decreases to a level which makes the alloy unusable for practical use, if the solution heat treatment is performed for improving the high temperature strength. The high temperature strength of the directionally solidified castings is evaluated as the strength in the solidified direction, because the direction wherein the main stress is applied is generally along the solidified direction. In this case, the high temperature strength, that is a strength in the solidified direction parallel to the grain boundary, improves in accordance with increasing solution of the .gamma.' phase. On the contrary, the grain boundary strength, that is a strength perpendicular to the grain boundary, and to the solidified direction, is decreased.
In accordance with the above findings, it is revealed that a simple addition of the chemical elements for grain boundary strength to the conventional single crystal alloy can be expected to improve the production yield of the products, but can not be expected to achieve a superior high temperature strength because the heating temperature for the solution heat treatment is decreased significantly. Regarding the conventional columnar grained alloys, the heating temperature for the solution heat treatment can not be increased further in view of problems of the incipient melting and decrease of grain boundary strength, and improving the high temperature strength more than the present status can not be expected.